Array.prototype.push. It modifies the array in place, which is asking for trouble. It also returns the array length, not the modified array, which is plain confusing.
Instead, prefer the array spread syntax[^1].
const original = ['cat', 'sat'] const modified = [...original, 'mat']
If you need to go old school, there's
const original = ['cat', 'sat'] const modified = original.concat(['mat'])
[^1]: Unless you're dealing with a massive array, where memory usage is an issue.